Hydroxy acids may be polycondensed in order to form polymers and some of them (glycolic acid (GA), lactic acid (LA), etc.) have been the subject of a resurgence of interest in recent years due to their bio-sourced nature.
The conventional synthesis of polyglycolide (PGA) adopts the same philosophy as the synthesis of polylactide. Firstly, a polycondensation of glycolic acid is carried out in order to obtain a low molecular weight oligomer. Then, at high temperature and low pressure, this oligomer is depolymerized with a view to distilling mainly the cyclic diester, glycolide. A relatively large number of purification steps follow in order to obtain an ultrapure (>99.90%) glycolide, which will then be subjected to a ring-opening polymerization (in accordance with a procedure similar to the polymerization of the caprolactone monomer, for example). The polymer obtained is linear and has a high molecular weight. Its cost price is high due to the cost of the depolymerization reaction and of the purification of the glycolide.
In fact, the first step of this process does generally not make it possible to obtain high enough molecular weights (this is why they were described as oligomers above), hence the need to resort to the subsequent steps. Takahashi et al., (Polymer, 41 (2000), 8725-8728: Melt/solid polycondensation of glycolic acid to obtain high molecular weight poly(glycolic acid)) describes a process that makes it possible to obtain higher molecular weight PGA by melt polycondensation in the presence of certain catalysts such as Zn acetate (hydrated or non-hydrated, the first being preferred). Although the molecular weights that are reported therein are relatively high, the Applicant, having reproduced the examples that were described therein and measured the melt viscosity (according to a procedure which will be described below), found very low values (of the order of 1 Pa·s) and observed an almost negligible melt strength, probably linked to the fact that the molecules obtained are linear.
However, melt strength is essential for processing operations of the type: blow molding of film, extrusion of a sheet or manufacture of a multilayer bottle by extrusion-blow molding. Since PGA has barrier properties that are comparable to those of EVOH or PVDC, its use in a PET/PGA/PET bottle structure would be very advantageous for packaging carbonated beverages that are oxygen-sensitive such as beer for example.
It is known from the literature to add polyfunctional agents to the polycondensation medium of certain polyesters in order to modify the structure thereof. Thus, Kim et al. (Journal of Applied Polymer Science, Vol. 80, 1388-1394 (2001)) describe the use of trimethylolpropane (TMP) in the polycondensation medium of succinic acid and 1,4-butanediol in order to form a branched PBS (poly(butylene succinate)). The Applicant has attempted to apply this teaching to hydroxy acids such as GA and LA to produce PGA and PLA, without succeeding in obtaining polymers that have a sufficient melt strength for the aforementioned types of processing operations.